1. Field of the Invention
Chlorine dioxide is a strong oxidizer and is widely used as a bleaching and/or disinfectant agent, with hundreds of tons being generated and used each day in the paper and water treatment industries. Chlorine dioxide is also used in considerably smaller quantities in treating agricultural produce and certain medical applications. Chlorine dioxide is well known as an algaecide, fungicide, germicide, deodorant, bleach, and general antiseptic. Currently, the equipment required for generating chlorine dioxide usually requires gaseous chlorine, sulfuric acid, or a combination of sodium hypochlorite (bleach) and acid with either sodium chlorite or sodium chlorate. Because one or more hazardous materials are typically required to generate the chlorine dioxide or produced as a byproduct, the use of chlorine dioxide has been somewhat limited. The cost of chlorine dioxide generating units and the need for trained personnel to operate and maintain the generating units has also hampered the wider utilization of chlorine dioxide. Accordingly, there exists a need for a method that is capable of readily and safely producing chlorine dioxide without requiring the use of more hazardous chemicals or generating them as reaction byproducts.
2. Background Art
Chlorine dioxide is a hazardous material. Pure chlorine dioxide is an oily, dark amber liquid and is extremely unstable at temperatures above xe2x88x9240xc2x0 C. Chlorine dioxide is also explosively unstable as a gas in concentrations greater than 10% by volume in air or at partial pressures above 76 mm Hg (1.46 psig). Above these levels chlorine dioxide may detonate if it contacts flammable organic solvents or other oxidizable materials. Chlorine dioxide is also pressure sensitive and may decompose violently if compressed, making it impractical to store or ship. Pure solutions of chlorine dioxide may also detonate if exposed to bright light or an ignition source such as heat, a spark, or an open flame. The upper boundary for safe chlorine dioxide concentrations in aqueous solutions is about 8 g/L at 30xc2x0 C., with most generating systems being designed to operate well below that limit, typically producing solutions having 1-3 g/L chlorine dioxide.
It is well known that chlorine dioxide is formed by reaction of sodium chlorite and an acid. For instance, U.S. Pat. No. 3,591,515 to Lovely teaches the use of various acidifying agents to generate chlorine dioxide at pH levels below 6 and form a dry powder fungicide. U.S. Pat. No. 4,330,531 to Alliger discloses a twin-compartment container for generating chlorine dioxide by reacting solutions of lactic acid and sodium chlorite. U.S. Pat. No. 4,585,482 to Tice, et al. discloses a biocidal composition that uses a chlorine dioxide-liberating compound and a hydrolyzable organic acid-generating polymer (such as a methylvinylether/maleic anhydride copolymer) for lowering the pH of the composition and slowly releasing the chlorine dioxide.
Chlorine dioxide is generally formed in one of two ways, either by reducing a chlorate ion (ClO3xe2x88x92) in an acidic medium according to reaction [1]:
ClO3xe2x88x92+2H++exe2x88x92xe2x86x92ClO2+H2O xe2x80x83xe2x80x83[1]
or by oxidizing a chlorite ion (ClO2xe2x88x92) according to reaction [2].
ClO2xe2x88x92xe2x86x92ClO2+exe2x88x92xe2x80x83xe2x80x83[2]
The choice of reducing agent for chlorine dioxide generation from chlorate has a great bearing on optimum reaction conditions, byproducts, and economics. Production from chlorite ion (ClO2xe2x88x92) is rather uneconomical. Indeed reaction [2] is reversible and chlorite is commonly synthesized from chlorine dioxide. Reducing agents typically used for producing chlorine dioxide from chlorate are sulfur dioxide (SO2), methanol (CH3OH), chloride ion (Clxe2x88x92), and hydrogen peroxide (H2O2). The associated half reactions are represented by reactions [3]-[6].
SO2+2H2Oxe2x86x92SO4xe2x88x922+4H++2exe2x88x92xe2x80x83xe2x80x83[3]
CH3OH+H2Oxe2x86x92HCOOH+4H++4exe2x88x92xe2x80x83xe2x80x83[4]
2Clxe2x88x92xe2x86x92Cl2+2exe2x88x92xe2x80x83xe2x80x83[5]
H2O2xe2x86x92O2+2H++2exe2x88x92xe2x80x83xe2x80x83[6]
Combining reaction [1] with reactions [3]-[6] produces reactions [7]-[10].
2ClO3xe2x88x92+SO2xe2x86x922ClO2+SO4xe2x88x922 xe2x80x83xe2x80x83[7]
4ClO3xe2x88x92+CH3OH+4H+xe2x86x924ClO2+HCOOH+3H2O xe2x80x83xe2x80x83[8]
ClO3xe2x88x92+Clxe2x88x92+2H+xe2x86x92ClO2+Cl2+H2O xe2x80x83xe2x80x83[9]
2ClO3xe2x88x92+H2O2+2H+xe2x86x922ClO2+O2+2H2O xe2x80x83xe2x80x83[10]
The byproducts formed are sulfate ion (SO42xe2x88x92), formic acid (HCOOH), chlorine (Cl2), and oxygen (O2). The acid equivalents required per mole of chlorine dioxide produced differ and are zero for sulfur dioxide, one for methanol, two for chloride, and one for hydrogen peroxide. Acid consumption is also influenced by the process conditions used in particular commercial designs.
In all systems, a side reaction may occur, reduction of chlorate to chloride according to reaction [11].
ClO3xe2x88x92+6H++6exe2x88x92xe2x86x92Clxe2x88x92+3H2O xe2x80x83xe2x80x83[11]
Steps must be taken to minimize this reaction by careful choice and control of reaction conditions. The basic mechanism of chlorine dioxide formation has been extensively studied and has previously been described in detail elsewhere by Haller and Northgraves in TAPPI (a publication of the Technical Association of the Pulp and Paper Industry) (April 1955) and Lenzi and Rapson in the Pulp Paper Magazine of Canada (1962). In each of the disclosed mechanisms, chloride plays a crucial role as evidenced by its presence in all chlorate-based reaction media and by the trace amounts of chlorine in the chlorine dioxide formed. No chlorine dioxide is formed if chloride ion is not present in the reaction medium. Chloride ion is introduced into the system by reduction of chlorate to chloride (reaction [11] above) or by addition of chloride in the feed. In a 1956 TAPPI article, Rapson proposed the following mechanism where chloride ion is the reducing agent.
HClO3+HClxe2x86x92HClO2+HClO xe2x80x83xe2x80x83[12]
HClO3+HClO2xe2x86x922ClO2+H2O xe2x80x83xe2x80x83[13]
HClO+HClxe2x86x92Cl2+H2O xe2x80x83xe2x80x83[14]
The formation of byproducts, other than those identified in reactions [7]-[10], is governed by the chlorate salt and acid selected. In all commercial processes, the chlorate salt used is sodium chlorate (NaClO3) and, to date, the most commonly used acids have been sulfuric (H2SO4) and hydrochloric (HCl). Consequently, the most common byproducts are sodium sulfate (Na2SO4) and sodium chloride (NaCl). Depending upon process conditions, sulfate is recovered as neutral crystalline sodium sulfate, sodium sesquisulfate (Na3H(SO4)2), or is dissolved in an acidic effluent. If hydrochloric acid is used, sodium chloride is recovered in a crystalline form or in an internally recycled solution.
Commercial chlorine dioxide generation systems can be broadly divided into atmospheric and sub-atmospheric processes. Atmospheric processes include the Mathieson, Solvay, and Rapson R2 processes which use sulfur dioxide, methanol, and sodium chloride, respectively, as the primary reducing agents. Each of the processes use air to strip and dilute the chlorine dioxide and have an overflow of spent sulfuric acid. In the 1950s, the Mathieson process was dominant, followed by the Solvay process. The Mathieson process was developed in 1950 by Olin-Mathieson Chemical Corporation and generated chlorine dioxide by reducing sodium chlorate with sulfur dioxide in the presence of sulfuric acid.
The Mathieson process chemistry generally follows reaction [7] above, i.e.
2NaClO3+SO2xe2x86x922ClO2+Na2SO4 xe2x80x83xe2x80x83[15]
However under low acidity conditions an unwanted side reaction, reflected in reactions [3] and [11] above, can considerably reduce the yield.
NaClO3+3SO2+3H2Oxe2x86x92NaCl+3H2SO4 xe2x80x83xe2x80x83[16]
In order to suppress this side reaction, an excess of sulfuric acid is typically fed to a Mathieson-type generator to create a 450-500 g/L acid concentration (9-10 N H2SO4). The acid (typically 2-2.5 tons of acid per ton of ClO2 produced) overflows from the generator and must be recovered and used elsewhere or, less preferably, neutralized and discharged. Some unreacted sodium chlorate also leaves the generator with the acid. The loss of the sodium chlorate and the contribution of reaction [16] typically limits the yield of a Mathieson-type generator to less than 90%.
The Solvay process uses methanol as the reducing agent and, like the Mathieson process, typically utilizes a 450-500 g/L sulfuric acid solution. The primary reaction, based on reaction [8], can be written as shown in reaction [17].
4NaClO3+H2SO4+CH3OHxe2x86x924ClO2+2Na2SO4+HCOOH+3H2O xe2x80x83xe2x80x83[17]
Both the Mathieson and Solvay processes are capable of producing chlorine dioxide solutions having low chlorine concentrations.
By the 1960s, the growing recognition of the crucial role of the chloride ion in chlorine dioxide synthesis processes had led to the increasing use of Rapson""s R2 process. The overall reaction in the R2 process, based on reaction [9] can be represented by reaction [18].
2NaClO3+2NaCl+2H2SO4xe2x86x922ClO2+Cl2+2Na2SO4+2H2O xe2x80x83xe2x80x83[18]
The R2 process, like the Mathieson process, is also subject to an unwanted side reaction, based on reactions [6] and [11], resulting in the production of chlorine according to reaction [19].
NaClO3+5NaCl+3H2SO4xe2x86x923Cl2+3H2O+3Na2SO4+xe2x80x83xe2x80x83[19]
As can be seen in reactions [18] and [19], the R2 process generates chlorine, typically in a 0.6:1 weight ratio with the desired ClO2. A portion of the chlorine, approximately 1 g/L of chlorine, usually remains in the chlorine dioxide solution with the balance being separated and used to produce sodium hypochlorite. Because the R2 reaction is much faster than the Mathieson or Solvay reaction, the process can be carried out in a single vessel. However, because the R2 reaction does not form the in situ acid of the Mathieson and Solvay processes, and because additional water is needed as the result of the addition of sodium chloride, the addition of approximately 4.5 ton of acid per ton of ClO2 is required for the R2 process, which limits its practicality for many industrial locations, particularly in areas where water consumption is an issue.
In response to the need to decrease the amount of waste acid produced, efforts were made to find methods of crystallizing or recycling the sodium sulfate from the waste acid. One solution was the application of an evaporator-crystallizer that could function as a chlorine dioxide generator with the steam and vacuum serving to control chlorine dioxide partial pressure. This led to the development of the R3/SVP process in which the basic R2 chemistry was performed under vacuum, at higher temperatures, and in the presence of proprietary catalysts, to achieve suitable production rates and yields with a reaction solution at acidities below 4.5 N H2SO4, the point below which neutral anhydrous sodium sulfate can be crystallized and filtered. The R3/SVP process also produced byproduct chlorine, typically in about the same 0.6:1 ratio as the R2 process, with ClO2 and about 2 g/L of Cl2 remaining in the chlorine dioxide solution and with the balance again being separated and used to produce chlorine water or hypochlorite.
As pulp mills decreased sodium and sulfur losses and also increased their use of chlorine dioxide, the amount of sodium/sulfur byproducts formed exceeded the pulp mills"" needs. Further, the use of byproduct chlorine to produce hypochlorite also became less attractive as mills worked toward obtaining both higher brightness pulps and suppressing or eliminating chloroform formation. These conditions and demands led to processes using hydrochloric acid as a replacement for part or all of the sulfuric acid in an R3 or SVP process (see reaction [20] below). The hydrochloric acid could either be purchased or made by burning byproduct chlorine with hydrogen in a hydrochloric acid burner, or alternatively, the chlorine could be reacted with sulfur dioxide and water to make a mixture of hydrochloric and sulfuric acids (see reaction [21] below). These changes, along with the partial replacement of sulfuric acid, significantly reduced the byproduct sodium sulfate and virtually eliminated the chlorine water and hypochlorite byproducts.
2NaClO3+2HCl+H2SO4xe2x86x922ClO2+Na2SO4+Cl2 xe2x80x83xe2x80x83[20]
Cl2+SO2+2H2Oxe2x86x922HCl+H2SO4 xe2x80x83xe2x80x83[21]
Hydrochloric acid processes can be operated independently or integrated with an onsite chlorate plant. The key reactions are the production of chlorine dioxide according to reaction [22] with sodium chlorate being produced by electrolysis of the byproduct salt according to reaction [23].
2NaClO3+4HClxe2x86x92ClO2+2Cl2+2H2O+2NaCl xe2x80x83xe2x80x83[22]
NaCl+3H2Oxe2x86x92NaClO3+3H2 xe2x80x83xe2x80x83[23]
The byproduct chlorine from the generator and supplemental chlorine are then reacted with the byproduct hydrogen from the chlorate reaction to produce HCl according to reaction [24], producing an overall stoichiometry for the integrated process, i.e., the combination of reactions [22]-[24], as reflected in reaction [25].
Cl2+H2xe2x86x922HCl xe2x80x83xe2x80x83[24]
Cl2+4H2Oxe2x86x922ClO2+4H2 xe2x80x83xe2x80x83[25]
This integrated process produces no sodium sulfate and requires chlorine input in a weight ratio of approximately 0.7:1 to that of the product ClO2 (reaction [24]) to make the necessary quantity of hydrochloric acid and help balance the NaOH/Cl2 needs. The integrated process does, however, increase the space and capital requirements compared with other alternative chlorine dioxide plants.
The interest in eliminating byproduct chlorine, decreasing byproduct sodium sulfate, and improving the overall efficiency and production rate in turn led to the development of an alternative methanol-based process. The overall reaction for this R8/SVP Methanol (MeOH) process can be represented by reaction [26].
3NaClO3+2H2SO4+0.80CH3OHxe2x86x923ClO2+Na3H(SO4)2+2.3H2O+0.8HCOOH xe2x80x83xe2x80x83[26]
The reaction represented in reaction [26] does not take into account the smaller amounts of methanol which typically leave the generator in the gas phase and/or dissolved in the chlorine dioxide solution. Some of the formic acid (HCOOH) reacts further according to reaction [27].
HCOOHxe2x86x92CO2+2H++2exe2x88x92xe2x80x83xe2x80x83[27]
Because formic acid has a similar vapor pressure to that of water, most of it is stripped from the generator. A typical 10 g/L ClO2 solution will also contain 0.2-0.9 g/L of CH30H, 1.7 g/L of CHOOH, 0.4 g/L of CO2, and 0.1 g/L of Cl2. Because the R8/SVP MeOH processes typically operate at acidities above 5 N, sodium and sulfate are recovered as sodium sesquisulfate (Na3H(SO4)2). This process has the advantages of virtually eliminating the chlorine byproduct (0.1 g/L of Cl2 in a 10 g/L solution of ClO2), producing less salt cake than the R3/SVP processes, increasing chlorine dioxide yield to over 95%, and improving production capacity. The residual chlorine is a product of the low concentration of chloride ions in the generator. If this low concentration of chloride ions is eliminated, the production of chlorine dioxide ceases, and the reactor enters a condition known as xe2x80x9cwhite-outxe2x80x9d in which the reactor generates a toxic white gas (comprising primarily chlorine and water vapor) and a grayish-white generator liquor. Maintaining lower acidities (approximately 5-7 N) helps maintain a sufficient concentration of chloride ions but results in decreased efficiency of methanol use. Alternatively, operation at higher acidities (approximately 8-10 N) leads to more efficient methanol use, but additional steps, including perhaps the addition of sodium chloride, are typically added to guard against a white-out condition.
When hydrogen peroxide is used as the reducing agent the overall reaction based on reaction [9] can be represented reaction [29].
2NaClO3+H2O2+H2SO4xe2x86x922ClO2+O2+2H2O+Na2SO4 xe2x80x83xe2x80x83[29]
The use of hydrogen peroxide as the reducing agent has the advantage of producing no byproduct chlorine and directly producing neutral sodium sulfate, but the relatively high cost of hydrogen peroxide has limited its widespread use.
Efforts have also been made to develop various electrochemical processes which can split the salt cake byproduct and/or the sodium chlorate feed into their respective acids and bases according to reactions [30] and [31].
Na2SO4+2H2Oxe2x86x922NaOH+H2SO4 xe2x80x83xe2x80x83[30]
NaClO3+H2Oxe2x86x92NaOH+HClO3 xe2x80x83xe2x80x83[31]
The electrochemical processes, however, are not yet cost-competitive with the more traditional chemical processes so their adoption tends to be driven more strongly by environmental pressures or restrictions relating to plant discharges and disposals.
Although not discussed here in detail, those of skill in the art will be familiar with a wide variety of alternative processes for generating chlorine dioxide including:
The Modified Mathieson process in which small amounts of sodium chloride (NaCl) were added to the reactants in the primary generator to improve the reduction efficiency of the chlorate and increase generator capacity.
The Hoist process which is a batch process similar to the Mathieson process with the significant difference being found in the solution concentrations and the batch-wise operation.
The Kesting Day-Fenn, or Day-Kesting, process utilizes hydrochloric acid (HCl) for reducing NaClO3 and is suitable for integration with an electrolytic plant for the production of chlorate.
The R1 process, the first of the xe2x80x9cRxe2x80x9d processes developed by Dr. Howard Rapson, relied on reacting sodium chlorate and sodium chlorite in a strong acid to form chlorine dioxide.
The R2 process used a mixture of NaClO3, H2SO4, and NaCl for reducing the chlorate, thereby eliminating the need for SO2. The R2 process, however, increased the formation of chlorine (Cl2) which was then absorbed (usually to form sodium hypochlorite (NaOCl)) after the ClO2 absorption tower.
The R2H process replaced NaCl and half of the H2SO4 used in the R2 process with hydrochloric acid (HCl).
The R3 process was another modified R2 process in which the reaction temperatures are
increased to boiling and concentrations increased strengthened to permit crystallization of sodium sulfate (Na2SO4) in the reaction vessel. This process, commercialized as the R3 process by Erco Ltd. (now a part of Sterling Pulp Chemicals, Ltd.) and as the SVP process by the Hooker Chemical Company (now Eka-Nobel), is also sometimes referred to as the xe2x80x9ceffluent-freexe2x80x9d process because since the byproduct removed is crystalline Na2SO4.
The R3H process, like the R2H process, replaced the NaCl and half of the H2SO4 of the R3 process with HCl.
The R5 process is basically the same as the R3 process with the exception that the NaCl and all of the H2SO4 are replaced with HCl, leaving HCl and NaClO3 as the only feed streams into the generator with the byproduct being crystalline NaCl suitable for reuse in chlorate manufacture. Further, the R5 process is distinguished from the R2 process by the higher operating temperatures and concentrations that permit the recovery of the crystalline byproduct and incorporates technology developed by Dow Chemical Canada.
The R6 process was an xe2x80x9cintegratedxe2x80x9d process used in conjunction with an electrolytic plant producing NaClO3. Variations of this basic xe2x80x9cintegratedxe2x80x9d process are also known as the Lurgi integrated process or the Chemetics integrated process.
The R7 process was another modification of the R3 process in which chlorine gas from the exit stream was reacted with SO2 to form a mixed acid of H2SO4 and HCl that is then returned to the generator. The only substantial byproduct of the R7 process is anhydrous Na2SO4.
The R8 process utilizes methanol as the reducing agent and produces an acid salt, sodium sesquisulfate [Na3H(SO4)2], as a byproduct. The sodium sesquisulfate must then be neutralized with caustic soda before recovery in the liquor system.
The R9 process is an extension of the R8 process in which the sodium sesquisulfate is diluted with water and separated into caustic soda and sulfuric acid in a membrane cell.
The R10 process is another extension of the R8 process in which the sodium sesquisulfate is diluted with both water and methanol following removal of sodium sulfate in a second filtration stage with the filtrate being returned to the generator.
The R11 process, uses hydrogen peroxide as the reducing agent for ClO2 generation. This process has seen limited use due to the higher operating costs associated with hydrogen peroxide.
The R12 process electrochemically converts sodium chlorate to a mixed feed of sodium chlorate and chloric acid (HClO3xc2x77H2O) which is fed to the ClO2 generator, which proportionally reduces sodium sulfate formation, while producing sodium hydroxide as a by-product.
The R13 process, uses chloric acid from the R12 process to produce ClO2 without the byproduct Na2SO4.
The SVP-Lite process is another methanol based process similar in some respects to the R8 process. The main difference between the SVP-Lite and R8 processes is the acid strength in the generator.
The SVP-HP process is similar to SVP-Lite except that it utilizes hydrogen peroxide rather than methanol for reducing the chlorate, is operated at subatmospheric pressures, and produces as its byproduct neutral sodium sulfate (Na2SO4).
The SVP-HPA process is an atmospheric process that is similar to the SVP-HP process in that it also utilizes hydrogen peroxide and produces Na2SO4 as a byproduct. In addition to the Na2SO4, the reactor also produces a spent acid stream.
The Lurgi process is an integrated process similar to the R6 and Chemetics processes in which an onsite electrolytic process is used to produce ClO2 from chlorine and water. The hydrogen produced is reacted with Cl2 to form HCl, which is, in turn, used in the process. Additional Cl2 is required to provide sufficient HCl for the process.
The Chemetics process is another integrated process similar to the R6 and Lurgi processes.
An alternative to the chlorate-based ClO2 synthesis is the chlorite-based synthesis. In the chlorite-based synthesis, a chlorite solution, typically sodium chlorite, is mixed with an acid to form an unstable chlorous acid, which in turn, disproportionates into chlorine dioxide according to reactions such as [32]-[34].
NaClO2+HClxe2x86x92HClO2+NaCl xe2x80x83xe2x80x83[32]
HClO2+3HClxe2x86x922Cl2+2H2O xe2x80x83xe2x80x83[33]
4NaClO2+2Cl2xe2x86x924ClO2+4NaCl xe2x80x83xe2x80x83[34]
The resulting overall reaction corresponds to reaction [35].
5NaClO2+4HClxe2x86x924ClO2+5NaCl+2H2O xe2x80x83xe2x80x83[35]
Although with simple mixing the initial chlorite is eventually consumed, the yield of ClO2 tends to be lower than expected, typically 60-80% depending on the starting proportions of the chlorite and acid reactants. Indeed, a concurrent reaction appears to be reflected by reaction [36].
4NaClO2+2HClxe2x86x922ClO2+NaClO3+3NaCl+H2O xe2x80x83xe2x80x83[36]
As a result of reaction [36] and other competing side reactions, the ClO2 produced tends to be impure and contain both chlorine and chlorate. The most common industrial applications rely on hydrochloric acid, sulfuric acid, and acetic acid, with hydrochloric acid being the most widely used. Catalysts for this family of reaction include sodium peroxide, potassium perborate, and cobalt sulfate. Typically yields are improved by running the synthesis with excess acid, typically about 300%, to increase the synthesis yields to levels approaching 100% (i.e., 4 moles of ClO2 produced from 5 moles of NaClO2). A much less frequently used synthesis described in Swiss patent 481,839 (1970) relies on a combination of powdered NaClO2 and powdered citric acid that are dissolved in 1.5 liters of water to produce a solution having about 5 g ClO2 in a solution of citric acid.
Chlorite can also be reacted with chlorine to produce ClO2 according to reaction [37].
2NaClO2+Cl2xe2x86x922NaCl+2ClO2 xe2x80x83xe2x80x83[37]
However, implementing this synthesis using solid chlorite and gaseous chlorine introduces contact problems such as the chlorite becoming coated with salt and local heating that increases the danger of an explosion. As a result, it is common to implement this reaction in solution under subatmospheric conditions to reduce the danger of explosion.
It is an object of the present invention to provide an improved method for the generation of aqueous chlorine dioxide solutions using as reactants only carbon dioxide gas and sodium chlorite that can implemented with less complex apparatus and with greater safety.
It is a further object of the present invention to produce chlorine dioxide in an aqueous system without adding corrosive ions such as sulfate and chloride.
It is a further object of the present invention to produce chlorine dioxide in an aqueous system without increasing the dissolved solids.
It is a further object of the present invention to produce chlorine dioxide in an aqueous system while reducing the potential for calcium carbonate scaling.
It is a further object of the present invention to produce chlorine dioxide in an aqueous system to reduce organic stack emissions.
It is a further object of the present invention to produce chlorine dioxide in an aqueous system for treatment of ground water contamination including, but not limited to, phenols, bacteria, manganese, and iron.
It is a further object of the present invention to produce chlorine dioxide in an aqueous system for conversion of hydrogen sulfide into hydrogen sulfate.
It is a further object of the present invention to provide a method for generating an aqueous solution of chlorine dioxide or a gaseous mixture including chlorine dioxide suitable for the treatment of agricultural products including leaf products such as tobacco, root crops such as potatoes, and other fruits and vegetables.